This invention relates to multiple drive axle land vehicles having a center differential between front and rear drive axles. In particular, the invention provides a novel three-position clutch mechanism for use with a fluid-operated bidirectional power means for transferring the clutch to an engaged or disengaged position from an intermediate, overrun position.
Heavy duty vehicles, such as six-wheel (three axle) drive truck tractors or the like are frequently provided with a power train which transfers driving power from a single power source to two or more axles. A center differential drive means may be employed for transferring the input power to the rear and front axles proportionately, the selected ratio depending upon static load distribution and anticipated dynamic distribution changes. The purpose of a center differential system is to compensate for rotational variations between front and rear axles and to avoid high drive train windup stresses which would otherwise occur.
Some prior vehicle drive assemblies have a central transfer case with front and rear output shafts which are differentially driven from a common input shaft. However, it is recognized that an unrestricted center differential drive system has a peak traction disadvantage when a substantial vehicle weight shift occurs that differs in ratio from the designated predetermined ratio. Similarly, traction problems can occur with a large road surface coefficient difference between front and rear axles. Chassis weight shift to the rear, such as automatically occurs when climbing a grade or towing a trailer load is another common condition. In this situation, the maximum vehicle drawbar pull will be prematurely limited because of a reduced reactive force on the front output of the center differential, and consequently, the front axle may spin out on the road surface prior to the rear.
To overcome the spinout condition which results from an unrestricted center differential, a common expedient has been to add a positive jaw clutch lock to temporarily eliminate differential action. Such a lock can be actuated by the driver or by automatic speed-sensitive controls. A disadvantage of this type of jaw clutch lock is that excessive use on high traction surfaces imposes driveline windup torques that can be additive to the normal drive torques, thereby reducing axle gear life. An improperly-trained operator may lock the center differential while climbing steep high-traction road surface grades with a fully loaded truck. Although this lock-up procedure assures that the vehicle will not spin out and come to a halt midgrade because of torque or surface variations, it results in severe wear which accompanies the extra torque strain.
Another device used to overcome vehicle center differential spinout is an overrunning clutch between the input and one or both of the differential outputs as disclosed in U.S. Pat. Nos. 2,796,941, 3,378,093, and 3,400,777 to Hill and 3,901,092 to Romick. Gear ratios and tire sizes are adjusted on the clutch output drive trains to the wheels so that the clutch overruns when the tires are not slipping. However, with tire slippage, lock-up occurs beyond a specified amount of differential output overrun. One disadvantage of the positive type of overrunning clutch, as used with a center differential and a "fast front" ratio match, is that excessive drive train torques can be imposed during panic brake stops by a transfer of rear brake torque to the front axle. During extreme deceleration, the rear axle tends to slow faster than the front, causing the overruning clutch to reverse direction, and the rear brakes retard the front wheels. Attempts to limit the peak front drive torque in this situation are illustrated in U.S. patents to Hill, U.S. Pat. No. 3,400,777 and U.S. Pat. No. 3,378,093 which disclose complex and costly friction disc, non-positive locks that apply differential braking on front axle overrun. A different attempt to correct the problem is illustrated in U.S. Pat. No. 3,901,092 to Romick, which discloses a completely separate friction-type, torque-limiting clutch in the front drive shaft. In addition to the cost, this latter system has the further disadvantage of forcing constant slippage and wear of the friction-type, torque-limiting clutch when in differential lockup, due to the planned ratio difference between front and rear axles to freewheel the overrunning clutch. My invention automatically disengages the overruning clutch during moderate-to-heavy braking, and thereby eliminates the problem.